The present invention relates to lighting systems including high intensity discharge lamps and electrical controls therefor.
High intensity discharge lamps in this specification are characterised in that they have a short arc length contained within an envelope, the envelope being filled with fill materials that may not be fully evaporated and hence have a low pressure when the lamp is cold and before the lamp has started conducting and when the lamp is operating and is hot, the said fill materials have a high pressure. High intensity discharge lamps are further characterised in that as a result of this increase in pressure of the fill material an ignition voltage required to start such lamps may increase sharply as the lamp becomes hot. For example a lamp with a cold ignition voltage of 2,000 volts may when hot require an ignition voltage of 30,000 volts to restart the lamp. High intensity discharge lamps frequently suffer from acoustic resonance, a phenomen whereby a frequency of the supply voltage excites a standing pressure wave within the lamp envelope.
Such electrical high intensity discharge lamps are constructed with a sealed envelope containing at least two electrodes for an electrical discharge, and are arranged to be used for lighting when an arc is established across the electrodes.
Such lamps have a high impedance before they are lit, and a low impedance while they are lit.
This characteristic means that in order to use such lamps in lighting systems it is necessary to combine the lamp with electrical controls matched to the lamp characteristic. Historically, the controls comprise a wound choke or ballast, which produces a back emf, which limits the current flow through the lamp while it is lit. To enable the lamp to become lit a starter is provided which produces a very high voltage across the lamp, which is normally sufficient to break down the resistance of the lamp and cause it to enter a conductive state. If a light fails to start on the first attempt, the starter will provide a further high voltage pulse across the lamp. This process continues until the lamp lights or the electrical power is removed from the circuit. It is this process that gives rise to the characteristic flicker of a discharge lamp that has failed to start when pulsed.
Although wound chokes are reliable, it is more energy efficient to provide controls comprising an electronic ballast. These electronic ballasts limit the current by generating a high frequency wave form which may provide for an intermittent power supply to the lamp.
As with a wound choke, when the controls comprise an electronic ballast to limit the current flowing through the lamp when it is lit, it is necessary to provide means for starting the lamp. While when a wound choke is used the starter can generate a high voltage by interrupting the current flow through the choke, there is insufficient stored energy in an electronic ballast to utilise it in this way. Hence it is necessary to incorporate means specifically to generate the high voltage required for starting the lamp. Known means to generate high voltage includes resonant circuits and suddenly discharged capacitor circuits. Known electronic controls having a self oscillating circuit operate at a frequency determined by the resonance of power handling components in the control circuit. A benefit of these self oscillating circuits is simplicity and low cost, however a disadvantage is that it is difficult to vary the operating frequency of such a control circuit as the operating frequency is determined solely by the values of fixed components, the values of which are determined by the power the circuit is arranged to control. Also known are electronic controls where the operating frequency is determined solely by a frequency generator such that the operating frequency can be arranged to be independent of the characteristics of power handling components in the circuit.
An oscillating control circuit where the frequency is determined by the characteristics of the power handling components is described in U.S. Pat. No. 5,341,067 to Nilssen. FIG. 1 of Nilssen""s Patent shows that the oscillation of the circuit depends on the characteristics of the capacitor 52, the inductor 51, and the saturable inductors 47 and 49. To prevent the destruction of the control circuit, Nilssen arranges for the capacitor 52 to be removed from the circuit with the lamp 26. Such removal of the capacitor interrupts the resonant circuit important to self-oscillation, and as a consequence the oscillation stops completely. Nilssen""s circuit is arranged to control and power a low pressure discharge lamp, such as fluorescent lamp, and would not be readily adaptable to operate a high intensity discharge lamp because of the very different starting and running requirements of high intensity discharge lamps, which require higher ignition voltages and operate with higher arc currents. Additionally the acoustic resonance phenomena exhibited by high intensity discharge lamps precludes operation in the frequency range specified by Nilssen.
UK Patent Application No. GB 2,226,463 to Yazdanian shows a high frequency electronic control utilising a frequency generator for use with a fluorescent lamp. Yazdanian discloses on page 9 to 11 the use of a series resonant circuit for lamp ignition, whereby there is a starting sequence in which the frequency sweeps from a frequency above the resonant frequency, through the resonant frequency to a lower frequency which is held for a dwell period, after which the frequency is increased again. Yazdanian states, on page 10 line 5, that a benefit of this is to assure that striking does take place for a variety of fluorescent lights irrespective of tolerances in the lamps and the control. This application does not disclose or teach the use of a limited frequency range as a means of discriminating between lamp power ratings.
A high intensity discharge lamp including a temperature dependent capacitor connected in series or in parallel with the lamp is shown in U.S. Pat. No. 4,134,042 to Philips. The Philips"" Patent describes in column 3, beginning at line 45, how on starting the lamp, an initial high impedance of the capacitor ensures the lamp rapidly reaches its operating condition, when the heat generated within the lamp heats the capacitor causing its capacitance value to decrease (and hence its impedance increases) causing a reduced current to flow through the lamp. As a result the lamp warms up quickly, but once hot is not over-powered. Philips does not teach the use of a thermally coupled capacitor to modulate the ignition of a lamp.
Once started discharge lamps generate heat and as a result discharge lamps can be more difficult to start after a period of use when the lamp is hot, than they are when they are first started from cold.
Since the controls tend to have a longer life than the lamp it is normal to incorporate the controls into a lighting system and to have the lamp easily replaceable. The lamp is normally provided with male contacts which when it is fitted in the lighting system make electrical connection with the electrical supply through female contacts in a lampholder. A hazard with the female contacts in the lampholder is that when the lamp is removed the controls will detect a very high impedance across the contacts, which will be similar to the condition which pertains before the lamp is lit. Hence the controls will try to start a lamp, by producing the necessary high voltages across the contacts in the lampholder. This means that the hazard from the exposed contacts is increased, since the supply voltage will typically be 240 v or 100 v, but the voltage produced by the starter circuit will be in the order of several thousand volts. Hence the hazard from accidental personal contact with the terminals is greatly increased by the starter. A further hazard arising from cost and space considerations means that the insulating parts in an electrical circuit comprising the controls, the lamp holder, lead wires interconnecting these parts and any intermediate connectors are frequently only rated for continuous use at the supply voltage. The nature of the insulating parts is such that the insulating parts can also withstand safely the much higher voltages that occur during starting, but only for a very short period. Hence when a lamp fails to start, or is absent from the lampholder, and the controls repeatedly try to start the lamp, it is known for the insulating parts to break down and conduct electricity. This has been identified as a cause of fire. Similarly the controls may incorporate control parts which have a limited ability to withstand the much higher voltages. Such control parts will be damaged if a lamp fails to start or is absent from the lampholder, and the controls repeatedly try to start the lamp.
To gain economy of scale, one size lampholder may accommodate several alternative power ratings of lamps. The lighting system manufacturer attaches a label to the lighting system informing a user of the correct lamp power rating to match the output of the controls and the heat dissipation capability of the lighting system. A problem arises when a user inserts a lamp of the incorrect power rating. There is a high risk that the lamp will overheat and explode with an attendant risk of personal injury and or damage to the lighting system and the controls. To overcome this problem some manufacturers have tried to prevent the accidental insertion of an incorrectly rated lamp in a lampholder for example by sizing the lampholder pins differently for different power ratings. A known example of preventing the insertion of a low power rated lamp into a socket intended to receive a higher power rated lamp, is that adopted by Philips with their MSR-HR lamp range, which consisted of three lamp powers, a 125 Watt, a 200 Watt and a 400 Watt. The 125 W lamp is provided with two large diameter pins to mount to a lampholder, while the 250 W lamp is provided with one small and one large diameter pin, and the 400 W lamp is provided with two small diameter pins, each pair of pins having the same pitch. Hence, it is possible incorrectly to insert a higher rated lamp into a socket intended to receive a lower rated lamp, but it is not possible to insert a lower rated lamp into a socket intended to receive a higher rated lamp. Hence a risk of explosive lamp failure is avoided. The disadvantage of this for the lighting system manufacturer is increased costs due to having to manufacture and stock a large range of lampholders.
A further problem for manufacturers of lighting systems is that any electrical interconnections forming an electrical circuit between the control and the lamp may have an interconnection reactance. Where an electronic ballast has a resonant circuit for starting the lamp, such an interconnection reactance may prevent the resonant circuit becoming properly resonant. Hence the lamp will not start satisfactorily. This problem can be alleviated by ensuring that such interconnection reactance is minimised. To minimise the interconnection reactance it is necessary to locate the control physically close to the lamp; this restricts the design of large lighting systems.
According to the invention there is provided a lighting system comprising at least:
(a) a high intensity discharge lamp, the high intensity discharge lamp comprising a sealed envelope containing at least two electrodes for an electrical discharge,
(b) a holder for the lamp arranged so that the lamp can be replaced
(c) an electronic control means for operating the lamp,
(d) a joint operating circuit included in and between the control means and the lamp,
(e) the control means having a variable frequency generator, the control means being so arranged to sweep over a pre-determined range of frequencies to produce by means of a resonant circuit a specific high frequency high voltage output for application to the joint operating circuit to ignite the lamp,
(f) the resonant circuit comprising at least an inductance in the control means and a capacitance in the joint operating circuit,
(g) said capacitance connected in parallel with a current path through the lamp,
(h) said capacitance mounted at least partly to the lamp.
In our co-pending patent application no PCT/GB99/038354 filed on the same date, we disclose a lighting system in which an adaptor and specifically appropriate circuitry is provided to ensure lamp type flexibility and a degree of safety. In the present invention the degree of safety has been enhanced by ensuring the system according to the preferred embodiment will only operate with a particular type of lamp.
Preferably the control is arranged so that when the joint operating circuit is in a resonant condition a high resonant voltage is generated. The high resonant voltage is arranged to be applied across the lamp. A value of the high resonant voltage is dependent on the type and size of the lamp that is to be started. For example the high resonant voltage would generally be between 500 volts and 50,000 volts.
A benefit of mounting the lamp capacitance in thermal proximity to the lamp is that a capacitance temperature will change so as to follow a change in a lamp temperature.
In an embodiment of the invention the lamp capacitance is mounted within a part of the envelope. In a preferred arrangement the envelope has at least two sealed compartments, a first compartment containing the discharge lamp elements, and a second compartment containing the lamp capacitance.
In an alternative embodiment of the invention the envelope is mounted to a cap, and the lamp capacitance is mounted within the cap.
Preferably the lamp capacitance has a temperature dependant coefficient of impedance. In an embodiment of the invention, the temperature dependant coefficient of impedance is negative. In an alternative embodiment of the invention the temperature dependant coefficient of impedance is positive.
The lamp may further comprise electrical supply connection means and the lamp capacitance is electrically connected across the electrical supply connection means and in a parallel current path to the lamp when the lamp is conductive.
A benefit of having the lamp capacitance in parallel with the lamp is that before the lamp is lit, when it has a high resistance and is effectively open circuit, the joint operating circuit includes the lamp capacitance. However once the lamp is lit, the resistance of the lamp is low, and the lamp effectively short circuits the lamp capacitance so that the joint control circuit then no longer effectively includes the lamp capacitance as an active part, and hence the joint operating circuit will not be resonant at the particular frequency.
If a capacitor providing the lamp capacitance is chosen so that the capacitor has a temperature dependant coefficient of capacitance, then the lamp capacitance will change with a change in the temperature of the lamp. Hence if the lamp capacitance has a negative temperature dependant coefficient of impedance, the value of impedance will fall with an increase in temperature arising from normal operation of the lamp. By careful selection of the value of capacitance and the temperature dependant coefficient of capacitance, the joint control circuit can be prevented from being resonant at the particular frequency when the lamp is hot. A benefit of this is that a non-resonant joint control circuit can not generate the high resonant voltage, and hence the circuit avoids unnecessary electrical stress on any component associated with a lamp supply circuit. This increases reliability and overall life of the lighting system.
A benefit of having such values for the lamp capacitance and the control reactance is that a reactance of interconnections between the control and the lamp may not prevent the joint control circuit from being resonant at substantially the particular frequency. The reactance of interconnections may be capacitive.
Preferably the value of impedance of the lamp capacitance is no less than one hundredth of the value of the impedance of the control reactance and no more than a hundred times the value of the impedance of the control reactance. More preferably the value of impedance of the lamp capacitance is no less than a tenth of the value of the impedance of the control reactance and no more than ten times the value of the impedance of the control reactance.
Preferably the value of impedance of the lamp capacitance is small in comparison with the capacitance of interconnections. Preferably where the capacitance of interconnections is predominantly capacitive, the impedance of the interconnection reactance is at least twice the impedance of the lamp capacitance and preferably the impedance of the lamp capacitance is at most four times the total impedance of the joint control circuit.
It will be noted that where an impedance of a reactance is measured, the impedance will change with frequency, and in this specification such frequency is substantially a resonant frequency of a joint control circuit that is arranged for the generation of high voltages for starting of a lamp.
Preferably the control is arranged to have at least two modes of operation, where the selection of the mode of operation of the control is dependent on the value of impedance of the lamp capacitance of the lamp fitted to the lighting system.
A benefit of the control having two modes of operation which are dependant on the value of impedance of the lamp capacitance of the lamp fitted to the lighting system is that where the value of impedance is affected by temperature, the mode of operation of the control will be dependant on the temperature of the lamp. Hence the control may be inhibited from operating in a first mode when the lamp is hot.
A further benefit is that when a lamp is absent from the lighting system, a value of impedance measured across the electrical supply connection means will be high as there is an open circuit across the electrical supply connection means, and hence the control may also be inhibited from operating in a first mode when the lamp is absent from the lighting system.
The control also comprises a variable frequency generator which may be arranged to operate sequentially so that at a cycle start time a start frequency is selected, and the frequency generator may over a sweep time period cause the frequency to fall to a dwell frequency which may then be held for a dwell time period and then increased to a sustained frequency which may be sustained for as long as the lamp continues to be lit.
Preferably the variable frequency generator is arranged so that the start frequency is higher than the dwell frequency, and preferably also higher than the sustained frequency. Preferably the variable frequency generator is also arranged so that the start frequency is above the particular frequency, and the dwell frequency is below the particular frequency.
In an embodiment of the invention, the variable frequency generator is arranged so that the sweep time period is preferably less than 250 ms (milli-seconds) and more preferably the sweep time period is less than 10 ms.
An advantage of a small value for the sweep time period is that the heating effect of any energy dissipated in the control is minimised so that cheaper components may be used in the manufacture of the control.
The dwell time period may be considerably longer than the sweep time period.
In an embodiment of the invention, the control is in a non-resonant mode of operation at the start frequency, and at the dwell frequency, and at the sustained frequency. Preferably a resonant mode occurs during the sweep time period as the frequency is falling. An effect of the resonant mode is to cause a high voltage to occur across the lamp. Preferably the high voltage is sufficient to cause the lamp to light.
A benefit of the dwell time period is that the lamp is given adequate time for the discharge to stabilise.
The variable frequency generator preferably is arranged so that the sustained frequency is preferably higher than the dwell frequency and more preferably within a stable operating frequency range of the lamp.
Preferably the resonant mode occurs within a stable operating frequency range of the lamp.
Preferably the control has control means for limiting the electrical current flowing through the lamp, so as to operate the lamp at its correct power rating.
According to a further aspect of the invention there is provided a lamp comprising a lamp capacitance mounted in thermal proximity to the lamp.
The above preferred embodiments of the one aspect of the invention are generally applicable to the further aspect of the invention.